Sometimes the patient with schistosomiasis experiences no trouble whatever; in other instances the suffering is very great.
—Sir Patrick Manson, 1898
Trematodes are flatworms, also called “platyhelminths” or “flukes.” They are divided into two major categories: the hermaphrodites and the schistosomes (Table 57–1). Of the many relationships that have developed between humans and helminths over millennia, perhaps the most destructive to our health and productivity is that forged by the trematodes. Typically, the adults live for decades within human tissues (for the hermaphrodites) and vasculature (for the schistosomes), where they resist immunologic attack and damage vital organs. Physicians and public health officers must understand trematode life cycles in order to make a meaningful difference in the lives of people impacted by these parasites.
TABLE 57–1General Characteristics of Trematodes ||Download (.pdf) TABLE 57–1 General Characteristics of Trematodes
|CHARACTERISTIC ||SCHISTOSOMES ||HERMAPHRODITES |
|Genus ||Schistosoma ||Paragonimus, Clonorchis, Opisthorchis, Fasciola, Fasciolopsis, Heterophyes/Metagonimus |
|Adult location in human body || |
| ||Bloodstream ||Tissue or intestines |
|Morphology || || |
|Adult ||Oral and ventral suckers ||Oral and ventral suckers |
| ||Blind gastrointestinal tract ||Blind gastrointestinal tract |
| ||Slender, worm like ||Flat, leaf like |
|Egg ||Nonoperculated ||Operculated |
|Biology || || |
|Sexes ||Separate ||Hermaphroditic |
|Intermediates ||One ||Two |
|Lifespan ||Long ||Long |
GENERAL FEATURES OF TREMATODES
Morphologically, trematodes are bilaterally symmetric, vary in length from a few millimeters to several centimeters, and possess two deep suckers from which they derive their name (“body with holes”). One surrounds the oral cavity, and the other is located on the ventral surface of the worm. These organs are used for both attachment and locomotion; movement is accomplished in a characteristic inchworm fashion.
Flukes move through tissue and vasculature with inchworm locomotion
The digestive tract begins at the oral sucker and continues as a muscular pharynx and esophagus before bifurcating to form bilateral ceca that end blindly near the posterior extremity of the worm. Undigested food is vomited through the oral cavity. The excretory system consists of a number of hollow, ciliated “flame” cells that excrete waste products into interconnecting ducts terminating in a posterior pore.
Trematodes are divided into two major categories, based on their reproductive systems: the hermaphrodites and the schistosomes (Table 57–1). The adult hermaphrodites contain both male and female gonads and produce operculated eggs (defined as having a lid). In contrast, the schistosomes have separate sexes, and the fertilized female deposits nonoperculated eggs. However, the two groups have similar life cycles. In both cases, eggs are excreted from the human host and—if they reach fresh water—hatch to release ciliated larvae called miracidia. These larvae find and penetrate a snail host specific for the trematode species. In this intermediate snail host, they are transformed by a process of asexual reproduction into thousands of tail-bearing larvae called cercariae, which are released ...